My advice would be to complete some quests and start building some units. Upgrade the unit facility to level 4, or you can also choose to build a 2nd one once your HQ reaches level 5. You will have a quest opened under "Missions" (click the actual mission star" -> World Map. This quest is for attacking your first level 1 renegade (+there is also one for killing 100 renegade units). Go ahead and complete this quest by opening it and then attacking the renegade with all of your units. I had produced 5-6 times extra units and ended up with 640 kills on my first attack.

Note: You will need to have a commander ready to deploy for this. It is possible that your commander is away with some of your army to gather oil for that achievement.